1. Field of the Invention
The present invention relates to a mechanism for automatically returning a movable swash plate to the neutral position in an axle driving apparatus which drives the axles thereof by a hydrostatic transmission (hereinafter referred to as an "HST") for steplessly changing the rotational speed of an engine, wherein resistance is given to the movable swash plate and pressure generated in a closed fluid circuit of the HST escapes therefrom when the movable swash plate returns to the neutral position, so as to prevent the vehicle from a sudden stop.
2. Related Art
Conventionally, an HST comprising a hydraulic pump and a hydraulic motor, which are fluidly connected through a closed fluid circuit, has a control arm which engages with a movable swash plate of the hydraulic pump. The quantity of oil discharged from the hydraulic pump can be changed in a stepless fashion by rotation of the control arm. The control arm is regularly biased by a neutral return spring, whereby the movable swash plate is automatically returned to its neutral position by the neutral return spring when an operator stops speed changing operation and releases the operational power. Accordingly, the movable swash plate suddenly returns to the neutral position when the operating force is released at a high speed position of the movable swash plate and operating oil which has smoothly circulated in the closed fluid circuit theretofore is blocked in the hydraulic pump so that the rotation of the hydraulic motor is suddenly stopped. In other words, a dynamic brake is applied. Therefore, a control arm is often provided with a shock absorber to prevent sudden returning to its neutral position, as disclosed in U.S. Pat. No. 5,094,077, for example.
A shock absorber which is provided to prevent the control arm from suddenly returning to its neutral position is disposed on the outside of the HST housing so as to enable mounting and exchange thereof However, a shock absorber which uses gas or fluid for shock absorption is heated by radiation of the housing which is heated by the rising temperature of the oil therein and is affected by the temperature of the outside air. The gas or fluid expands or contracts because of such a change of temperature so that operation of the shock absorber differs according to temperature or, in some cases, the neutral position can not be fixed at a high or a low temperature.
Further, a shock absorber mounted on the outside of the housing requires space for it to be mounted which can restrict the shape of the axle driving apparatus. As a result, the entire axle driving apparatus becomes larger. Also, because the shock absorber is mounted outside of the housing, the movable portion of the shock absorber is conventionally covered with a protector such as a rubber boot to avoid penetration of water, dust or other foreign matter, and therefore, requires an increase in the number of parts and in the cost.